Currently, asymmetrical a-BPDA is being prepared from o-xylene or o-phthalate via an oxidative coupling reaction, which essentially yields a mixture of 3,3′,4,4′-biphenyl dianhydride (s-BPDA) and a minor product (2-6%) of a-BPDA. Consequently, a-BPDA is being produced in limited quantity and therefore is not commercially available in sufficient amounts, despite an enormous interest in preparing polyimides using a-BPDA. This invention discloses alternative and more efficient processes for exclusively producing asymmetric 2,3,3′,4′-biphenyl dianhydride (a-BPDA), 2,3,3′,4′-benzophenone dianhydride (a-BTDA) and 3,4′-methylendiphthalic anhydride (a-MDPA). The asymmetric 2,3,3′,4′-biphenyl dianhydride (a-BPDA) has received more attention, because this dianhydride can produce polyimide resins having low-melt viscosities and higher glass-transition temperatures (Tg) than the symmetrical 3,3′,4,4′-biphenyl dianhydride (s-BPDA).
The prior art (U.S. Pat. No. 3,940,426, UBE Industries) process for making a-BPDA relies on oxidative coupling of o-xylene or methyl o-phthalate with an organic acid salt of palladium under oxygen pressure to produce symmetrical and unsymmetrical intermediates which are oxidized and cyclodehydrated to form a mixture of a-BPDA and s-BPDA. This mixture requires the additional process of separating the two isomers.
U.S. Pat. No. 4,294,976 discloses a process for preparing a mixture of biphenyltetracarboxylic acids (3,3′,4,4′-isomer, 2,3,3′,4′-isomer and 2,2′,3,3′ isomer) via an oxidative coupling of either o-xylene or o-phthalate in the presence palladium catalyst, followed by hydrolysis. The mixture of isomeric biphenyltetracarboxylic acids were then subjected to fractionally recrystallization to obtain each of the corresponding 2,3,3′,4′-(minor amount) and 3,3′,4,4′-(major amount) and minute amount of 2,2′,3,3′-biphenyldianhydrides.
U.S. Pat. No. 4,958,002 teaches a dehydration process to obtain 3,3′,4,4′-biphenyl dianhydride after the corresponding 3,3′,4,4′-biphenyltetracarboxylic acid was isolated from 2,3,3′,4′-biphenyltetracarboxylic acid. U.S. Pat. No. 5,258,530 (Mitsibishi) describes a coupling reaction of phthalic anhydride directly to a mixture of 2,3,3′,4′-(major) and 2,3,3′,4′-(minor) biphenyl dianhydrides. U.S. Patent Publication No. 0088120 A1 (2003) discloses a process for producting predominately 2,3,3′,4′-biphenyl dianhydride (a-BPDA) over a minor amount of 3,3′,4,4′-biphenyldianhydride (s-BPDA) using a palladium and copper catalyst along with bidentate ligand. These prior art processes yield mixtures of asymmetrical 2,3,3′,4′-BPDA together with symmetrical 3,3′,4,4′-BPDA, which then requires the separation of these isomers. In comparison, this invention discloses an asymmetrical coupling of substituted o-xylenes to provide asymmetrical 2,3,3′,4′-BPDA exclusively without a mixture or contamination by the symmetrical 3,3′,4,4′-BPDA or 2,2′,3,3′-BPDA.